Decisions like those should always be made at the start of the project, and be made transparent to all involved. If the personnel are going to be terminated at the end of a project, then their involvement is contractual and should be written up/treated/compensated as such. If they are going to be kept on but transferred to another project, that should be mentioned as well. Any change to these agreements should be made known to the resources AT THE TIME THE CHANGE IS DECIDED UPON.
To do otherwise would be unfair to the project team, and would be breaking faith with your employees.